A stainless steel containing B is applied as a thermal neutron control rod as well as a thermal neutron shielding material by utilizing an excellent thermal neutron absorption effect of boron (B). In general, spent nuclear fuels that were generated at a nuclear power plant is stored in a pool within a site of a nuclear power plant until being reprocessed at the reprocessing plant. Since there is a need that the spent nuclear fuels shall be stored in a limited space as much as possible, there is a tendency that a B content in a stainless steel containing B is further increased and a wall thickness of source material for components gets thinner.
Austenitic stainless steels have excellent resistance to corrosion owing to the formed passive film on the surface, and further an addition of B into it improves the electric resistance characteristic, thus enabling to be used for high conductivity electric device requiring further corrosion resistance. As an exemplified usage of high conductivity electric device requiring further excellent corrosion resistance as well as low electric resistance at contact portion, there is a separator for fuel cell that generates direct current electric power by utilizing hydrogen and oxygen.
In hot working of a stainless steel containing B, the repetition of heating blooms by reheating furnace along with forging and rolling serves to prevent the temperature of a workpiece material from falling down, thus enabling hot workability to be secured. Since, as the content of B increases, the hot workability is reduced, it becomes necessary to undergo hot working process without lowering the temperature of a workpiece material, thus resulting in increasing a number of repetition of heating and hot working significantly. Accordingly, the increase of B content as well as metal forming to thin sheet amounts to the increase of production cost.
To deal with above-mentioned problem, various methods have long been attempted. For instance, in the Japanese Patent Publication No. 04-253506, there is disclosed a hot rolling method of steel slab that a workpiece material prepared in such a way that both side edge portions of a base metal comprising an austenitic stainless steel containing B of 0.3–2.0 mass % are covered with a steel member (hereinafter, may be referred to as “side plate”) having lower deformation resistance by welding and are subjected to finishing rolling at the temperature of (53 B+700)° C. (where B denotes B content (mass %)) or higher, thus enabling to prevent the generation of edge cracking during rolling.
However, in this method, it is a prerequisite that the preparation of groove configuration with high accuracy for a frame material is carried out and welding itself must be performed so that the frame material would not fall apart during hot working. Therefore, in general, in applying this method to an ingot (cast ingot) or a bloomed and forged slab having a thickness of 80 mm or more, huge welding time is needed.
Also, in case of rolling a wide plate exceeding 1000 mm in width, it is likely to run into the difficulty in securing above temperature for finishing rolling, thus resulting in difficulty to practically prevent the generation of edge cracking.
Further, in case a base metal is welded with a side plate, B (boron) diffuses from a base metal to the weld metal and forms a mixture therein. Due to this, there is an occasion that cracks generate at the weld portion during rolling, causing rolling trouble and or leading to edge cracking developed from the initiation site of said cracks at the weld portion.
In the Japanese Patent Publication No. 2001-239364, there is disclosed a hot rolling method that, in hot rolling an austenitic stainless steel slab containing B of 0.3–2.5 mass %, a protecting layer of 3 mm or more in thickness being formed by overlay welding and comprising a stainless steel containing Ni of 4% or less and B of 0.1–0.4% is disposed at side edge portions of the slab.
In the Japanese Patent Publication No. 09-269398, there is disclosed a hot rolling workpiece material formed by a slab of an austenitic stainless steel containing B of 0.6–2.0 mass % and a layer by overlay weld formed at both side edge portions of said slab, a layer by overlay weld comprising a δ ferrite content of 3–12 volume %, and a B content of 0.3 mass % or less, and a thickness of 3 mm or more.
However, in this overlay welding method, a number of pass in welding increases to secure an adequate weld thickness so as to prevent the generation of cracks, thus ending up to an increase of welding time. Moreover, when weld cracks happen to generate, these cracks likely behave as an initiation site, resulting in the generation of edge cracking. Thus, it is very unlikely to completely prevent edges cracking from generating.